The present invention relates generally to communication systems, and more particularly to system and method for improving the signal-to-quantization noise ratio of an orthogonal frequency division multiple access digital modulator.
In recent years, communications designs increasingly rely on the orthogonal frequency division multiple access (OFDMA) mode for wireless communications due to its superior performance in multi-path fading wireless channels. In a multi-carrier transmission environment, the data stream is split into multiple parallel data streams of reduced rate, each of them transmitted on a separate frequency or sub-carrier, and each sub-carrier must be spaced away from another such that its signal does not interfere with the signal of another. The OFDMA is a special case of multi-carrier transmission that permits sub-channels to overlap in frequency without mutual interference. Because sub-channels can now overlap without interference, an increased spectral efficiency is achieved. In other words, OFDMA exploits signal processing technology to obtain the most cost-effective means of implementation, wherein multiple users can be supported by allocating each user a group of sub-carriers, without fearing that sub-carriers themselves would interfere with each other. The theories of OFDMA mode are well known and are well explained in “Multi-carrier Digital Communications: Theory and Applications of OFDM” by Ahmad R. S. Bahai et al. 2nd Ed., Springer, October 2004. OFDMA technology has been recommended and successfully implemented in other wireless standards such as digital video broadcasting (DVB) and wireless local area networking (WLAN).
As wireless technology such as the OFDMA mode continues to advance, the minimization of cost and power consumption for communication devices is much needed. In order to minimize the size and power consumption of the chips within a communication system, an efficient implementation of algorithms in the hardware architecture is required. Thus, it is of paramount importance to minimize the word-length of the data as long as the desired precision constraints are respected.
One of the most commonly used criteria for evaluating whether the desired precision constraints are met is the signal-to-quantization noise ratio (SQNR). In a typical wireless system, a first stage to perform such an evaluation is the estimation of the dynamic range of the data in order to determine the word-length of their integer part. Then, the word-lengths of the data are optimized according to one or more desired SQNR constraints. The achievement of a second stage is based on the availability of a tool allowing the evaluation of quality of the implementation through the determination of the SQNR at the output of the system.
However, conventional OFDMA digital modulators use fixed-point inverse fast Fourier transform (IFFT) devices, which do not properly allow the detection of the number of active sub-carriers and scale. As a result, the traditional fixed-point IFFT device is required to have a higher dynamic range, thus requiring a more complex fixed-point arithmetic system and/or additional dynamic scaling circuitries. These additional complexity yields an undesired level of SQNR. The mechanics of these conventional IFFT devices are well known and are explained in detail in “The Fourier Transform & Its Applications” by Ronald N. Bracewell. 3rd Ed., McGraw-Hill Science, June 1999.
Therefore, it is desirable to devise a system and method for implementing a fixed-point IFFT design that has the ability to detect active sub-carriers and scale accordingly in order to improve the SQNR of an OFDMA modulator.